Neptunium and Manganese Biocycling in Nuclear Legacy Sediment Systems

11 Neptunium and Manganese Biocycling in Nuclear Legacy Sediment2 Systems3 Clare L. Thorpe1, Katherine Morris1, Jonathan R. Lloyd1, Melissa A. Denecke2,4 Kathleen A. Law2, Kathy Dardenne3, Christopher Boothman1, Pieter Bots1, Gareth T.5 W. Law2*67 1 Research Centre for Radwaste Disposal and Williamson Research Centre for Molecular8 Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The9 University of Manchester, Manchester, M13 9PL, U.K.10 2 Centre for Radiochemistry Research, School of Chemistry, The University of Manchester,11 M13 9PL, U.K.12 3 Karlsruhe Institute of Technology, Institut Fur Nukleare Entsorgung, D-76021 – Karlsruhe,13 Germany. 14 215 *Corresponding author: Email: [email protected]; Tel: +44 (0)161 275 16 1428Abstract 17 Understanding the behaviour of the highly radiotoxic, long half-life radionuclide neptunium18 in the environment is important for the management of radioactively contaminated land and19 the safe disposal of radioactive wastes. Recent studies have identified that microbial20 reduction can reduce the mobility of neptunium via reduction of soluble Np(V) to poorly21 soluble Np(IV), with coupling to both Mn- and Fe(III)- reduction implicated in neptunyl22 reduction. To further explore these processes Mn(IV) as δMnO2 was added to sediment23 microcosms to create a sediment microcosm experiment “poised” under Mn-reducing24 conditions. Enhanced removal of Np(V) from solution occurred during Mn-reduction, and25 parallel X-ray absorption spectroscopy (XAS) studies confirmed Np(V) reduction to Np(IV)26 commensurate with microbially-mediated Mn-reduction. Molecular ecology analysis of the27 XAS systems, which contained up to 0.2 mM Np showed no significant impact of elevated28 Np concentrations on the microbial population. These results demonstrate the importance of29 Mn cycling on Np biogeochemistry, and clearly highlight new pathways to reductive30 immobilisation for this highly radiotoxic actinide. 331 Introduction32 Internationally, deep geological disposal is being considered as the long-term management33 and disposal option for higher activity radioactive wastes (HAW). A fundamental knowledge34 of reactions between radionuclides and geomedia is essential to underpin the safety case for35 geodisposal. Neptunium is a key risk-driving radionuclide in HAW due to its long half-life36 (237Np t½ = 2.1 x 106 years), ingrowth from 241Am, high radiotoxicity, and relatively high37 solubility as Np(V). Indeed, Np is potentially the most mobile transuranic species in38 environments pertinant to deep geological disposal (e.g. Choppin and Stout, 1989; Kaszuba39 and Runde, 1999; Lloyd et al., 2000; Choppin, 2007; Law et al., 2010); further, Np is a40 persistant contaminant at or near nuclear sites (e.g. Cantrell, 2009; Morris et al., 2000;41 Stamper et al., 2013).42 Neptunium is redox active and it’s environmental mobility can be affected by the43 biogeochmistry and redox conditions in the subsurface (Kaszuba and Runde, 1999; Lloyd et44 al., 2002; Choppin, 2007; Law et al., 2010). Under oxidising conditions Np is stable in+ 45 solution as the soluble neptunyl cation, NpO2 , whilst under anaerobic conditions Np can be46 reduced to poorly soluble Np(IV) species (Kaszuba and Runde, 1999; Moyes et al., 2002;47 Llorens et al., 2005; Law et al., 2010; Bach et al., 2014). In the subsurface, microbial48 respiration can induce anaerobic conditions under which metals and radionuclides can be49 reduced (Lloyd and Renshaw, 2005). The development of bioreducing conditions is50 increasingly recognised as likely to be significant in the deep subsurface around a geological51 disposal facility (Pedersen, 2000; Fredrickson and Balkwill, 2006; Rizoulis et al., 2012;52 Williamson et al., 2013; Behrends et al., 2012), and is the basis for remediation of53 contaminated land where problematic radionuclides (e.g. Tc and U) may be reduced either54 enzymatically or indirectly via interactions with reduced species (e.g. Fe(II): Lloyd et al.,55 2002; Lloyd, 2003; Gadd, 2010; Newsome et al., 2014). 456 The ability of microorganisms to enzymatically reduce Np(V) to Np(IV) has been57 demonstrated in pure culture experiments (Lloyd et al., 2000; Icopini et al., 2007) although58 some microorganisms are unable to facilitate enzymatic Np(V) reduction (Songkasiri et al.,59 2002; Renshaw et al., 2005). In contrast, Gorman-Lewis et al., (2005) inferred potential non-60 enzymatic reduction of Np(V) to explain trends in sorption / desorption experiments61 conducted with Bacillus subtilis. Toxicity effects on selected metal-reducing bacteria are also62 of interest as studies with indigenous microorganisms highlight the tolerence of63 microorganisms to mM concentrations of Np (Law et al., 2010; Ams et al., 2013), whilst in64 pure culture experiments no toxicity effects were observed at Np concentrations less than65 2mM (Ruggiero et al., 2005). In sediment systems, reductive immobilisation of Np(V) to66 Np(IV) has been observed during development of sediment anoxia with microbial metal67 reduction implicated in the reaction and with indirect (abiotic) reduction by Fe(II) shown to68 be possible (Law et al 2010).69 Manganese is ubiquitous in soils and rock forming minerals and therefore, although Np70 interactions with Mn minerals have been studied previously (Wilk et al., 2005), a deeper71 understanding of Np(V) behaviour during early metal reduction (Mn- and Fe(III)-reduction)72 is essential in understanding its environmental behaviour in both deep and shallow subsurface73 environments. In addition, the potential importance of Mn in environmental actinide74 chemistry is increasingly recognised with Mn linked to both Pu and U cycling (Powell et al.,75 2006; Hu et al., 2010; Wang et al., 2013; Wang et al., 2014). Here we examine the behaviour76 of Np in sediment systems amended with labile Mn(IV) (δMnO2) to allow microcosms to77 develop a period of extended or “poised” Mn reduction (Lovley and Philips, 1988). As well78 conducting experiments at low Np concentrations, we also collected XAS data from parallel79 experiments run at higher concentrations of Np. This allowed assessment of Np speciation80 and local-coordination under defined biogeochemical conditions. Finally, 16S rRNA gene 581 analysis was performed to assess the response of the indigenous microbial communities to82 elevated Np concentrations.8384 Experimental Section 85 Safety86 Neptunium (237Np) is a high radiotoxicity alpha-emitting radionuclide with beta/gamma87 emitting progeny. Work can only be conducted by trained personnel in a certified, properly88 equipped radiochemistry laboratory, following appropriate risk assessment. The possession89 and use of radioactive materials is subject to statutory control.9091 Sample Collection92 Sediments were collected from an area located ~ 2 km from the Sellafield reprocessing site in93 Calder River Valley, Cumbria (Lat 54°26’30 N, Long 03°28’09 W). Sediments were94 representative of the Quaternary unconsolidated alluvial flood-plain deposits that underlie the95 Sellafield site (Law et al., 2010) and were collected in sterile containers, sealed, and stored at96 4 ºC prior to use (< 1 month).97+ 98 Bioreduction Microcosms with Low NpO2 Concentrations 99 Sediment microcosms (10 ± 0.1 g Sellafield sediment, 100 ± 1 ml groundwater; in triplicate)100 were prepared using a synthetic groundwater representative of the Sellafield region (Wilkins101 et al., 2007) but with added nitrate and manganese (2 mM NaNO3, 2 mM δMnO2) and with a102 total 0.17 mmols of bioavailable Fe(III) in the sediment. Sodium acetate was also added in103 stoichiometric excess (10 mM) as an electron donor, the groundwater was sterilised104 (autoclaved for 1 hour at 120 ºC), purged with filtered 80 % / 20 % N2 / CO2, and pH adjusted105 to pH 7 (via drop-wise addition of 0.5 M HCl or 1M NaOH). Sediments and sterile 6106 groundwater were then added to sterile 120 ml glass serum bottles (Wheaton Scientific, USA)237 + 107 and sealed with butyl rubber stoppers using aseptic technique. Neptunium, as NpO2108 (20 Bq ml-1; 3.2 µM; oxidation state verified by UV-Vis analysis) was then spiked into each109 microcosm; thereafter, the microcosms were incubated anaerobically at 21 ºC in the dark for110 38 days. Throughout the incubation, sediment slurry was periodically extracted using aseptic111 technique, under an O2-free, Ar atmosphere. The sediment slurry was centrifuged (15,000 g;112 10 minutes) to separate sediment and porewater samples and ~ 0.5 g of sediment was stored113 at - 80 ºC for later microbiological characterisation.114115 Geochemical Analyses116 During microcosm sampling, total dissolved Mn and Fe concentrations were measured with117 standard UV-vis spectroscopy methods on a Jenway 6715 spectrophotometer (Lovley and- 2- 118 Philips, 1987; Goto et al., 1997; Viollier et al., 2000). Aqueous NO3 , SO4 , ammonium and119 acetate were measured by ion chromatography (Dionex ICS5000). Total bioavailable Fe(III)120 and the proportion of extractable Fe(II) in the sediment was estimated by digestion of 0.1 g of121 sediment in 5 ml of 0.5 N HCl for 60 minutes followed by the ferrozine assay, with and122 without added hydroxylammonium chloride (Lovley and Phillips, 1987; Viollier et al., 2000).123 The pH and Eh were measured with an Orion 420A digital meter and calibrated electrodes.124 Standards were routinely used to check the reliability of all methods and typically, calibration125 regressions had R2 ≥ 0.99. The elemental composition and bulk mineralogy of the sediment126 were determined by XRF (Thermo ARL 9400) and XRD (Philips PW 1050). Total organic127 carbon and total inorganic carbon were determined on a LECO CR-412 Carbon Analyser.128 The total 237Np concentration in solution was measured by ICP-MS (Agilent 7500cx) using129 232Th as the internal standard. 130 7131 XAS Experiments132 Experiments were prepared to allow direct determination of Np speciation and local133 coordination environment in sediments under different geochemical conditions using X-ray134 Absorption Spectroscopy (XAS). Here, the elevated concentration of Np required for direct+ 135 spectroscopic characterisation (0.2 mM Np(V) as NpO2 in 0.07 M HCl) was added to136 microcosms containing 1g of sediment and 10 ml of groundwater that were poised at oxic,137 nitrate-, Mn-, Fe(III)-, and sulfate-reducing conditions, respectively. After Np(V) addition,138 the microcosms were left to incubate for 1 week in the dark at 7 ºC prior to geochemical139 sampling and subsequent freezing at – 80 ºC. Two additional Mn-reducing XAS systems+ 140 were also established where sediments had 2 mM of δMnO2 added: (i) 0.2 mM NpO2 was141 added to an oxic microcosm that was then left to progress to Mn-reducing conditions142 (verified by the presence of Mn in porewaters and the absence of detectable 0.5 N extractable143 Fe(II) in sediments) before freezing at -80 oC, and (ii) a parallel Mn-reducing microcosm144 (again with no detectable 0.5 N extractable Fe(II) in sediments) that was sterilised by+ 145 autoclaving (1 hour at 120 ºC) prior to the addition of 0.2 mM NpO2 , and which was frozen146 at -80 oC 2 days after Np(V) addition. For XAS analysis, sediment samples were defrosted,147 centrifuged, and ~ 0.5 g of sediment was packed (under anaerobic atmosphere if necessary)148 into airtight sample containers which were then triple contained and frozen until analysis.149 XAS analysis was conducted at the INE Beamline for Actinide Research at the ANKA150 synchrotron, Karlsruhe, Germany. Neptunium LIII-edge spectra (17610 eV) were collected in151 fluorescence mode by a 5 element solid-state Ge detector. Parallel K-edge measurements152 from a Zr foil were recorded for energy calibration. XANES data were collected for all153 samples and EXAFS data were collected for selected samples. Background subtraction, data154 normalization, and fitting to EXAFS spectra were performed using the software packages 8155 Athena and Artemis. The XANES edge-jump was tied to unity. Modeling of the EXAFS156 data in k3 range was completed between 3 and 9.5 Ǻ-1.157158 Microbial Community Analysis159 Samples from an oxic sediment, and a Mn-reducing sample were taken from both low Np160 (20 Bq ml-1; 3.2 μM) and high Np (1.3 kBq ml-1; 0.2 mM) microcosms and analysis was161 performed using PCR-based 16S rRNA gene analysis.162163 Ribosomal Intergenic Spacer Analysis164 DNA was extracted from Np containing microcosm samples (200 µl) using a PowerSoil165 DNA Isolation Kit (MO BIO Laboratories INC, USA). The 16S-23S rRNA intergenic spacer166 region from the bacterial RNA operon was amplified as described previously using primers167 ITSF and ITSReub (Cardinale et al., 2004). The amplified products were separated by168 electrophoresis in tris-acetate-EDTA gel. The DNA was stained with ethidium bromide and169 viewed under short-wave UV light. Positive microbial community changes identified by the170 RISA justified further investigation by DNA sequencing of 16S rRNA gene clone libraries. 171 Amplification of 16S rRNA Gene Sequences172 A fragment of the 16S rRNA gene (approximately 1490 b.p.) was amplified from samples173 using the broad-specificity primers 8F (Eden et al., 1991) and 1492R (Lane et al., 1985). PCR174 reactions were performed in thin-walled tubes using a BioRad iCycler (BioRad, UK). Takara175 Ex Taq Polymerase (Millipore, UK) was used to amplify DNA from the sample extract. The176 PCR amplification protocol used with the 8F and 1492R primers was: initial denaturation at177 94 ºC for 4 minutes, melting at 94 ºC for 30 seconds, annealing at 50 ºC for 30 seconds,178 elongation at 72 ºC for 3 minutes and 35 cycles, followed by a final extension step at 72 ºC 9179 for 5 minutes. The purity of the amplified products was determined by electrophoresis in tris-180 acetate-EDTA (TAE) gel. DNA was stained with ethidium bromide and viewed under short-181 wave UV light using a BioRad Geldoc 2000 system (BioRad, UK). 182 Cloning183 PCR products were purified using a QIAquick PCR purification kit (Qiagen, UK) and ligated184 directly into a cloning vector containing topoisomerase I-charged vector arms (Agilent185 Technologies, UK) prior to transformation into E. coli competent cells expressing Cre186 recombinase (Agilent Technologies, UK). White transformants that grew on LB agar187 containing ampicillin and X-Gal were screened for an insert using PCR. Primers were188 complementary to the flanking regions of the PCR insertion site of the cloning vector. The189 PCR method was: an initial denaturation at 94 ºC for 4 minutes, melting at 94 ºC for 30190 seconds, annealing at 55 ºC for 30 seconds, extension at 72 ºC for 1 minutes and 35 cycles,191 followed by a final extension step at 72 ºC for 5 minutes. The resulting PCR products were192 purified using an ExoSap protocol, here 2 μl of ExoSap mix (0.058 μl Exonuclease I, 0.5 μl193 Shrimp Alkaline Phosphatase and 1.442 μl H2O) was added to 5 μl of PCR product and194 incubated at 37 ºC for 30 minutes followed by 80 ºC for 15 minutes.195 DNA Sequencing and Phylogenetic Analysis196 Nucleotide sequences were determined by the dideoxynucleotide method. An ABI Prism197 BigDye Terminator Cycle Sequencing Kit was used in combination with an ABI Prism 877198 Integrated Thermal Cycler and ABI Prism 377 DNA Sequencer (Perkin Elmer Applied199 Biosystems, UK). Sequences (typically 900 base pairs in length) were analysed using200 Mallard (Ashelford et al., 2006) to check for presence of chimeras or sequencing anomalies.201 Operational taxonomic units (OTU) were determined at a 98 % sequence similarity level202 using Mothur (Schloss et al., 2009). The individual OTU sequences were analysed using the 10203 sequencing database of known 16S rRNA gene sequences provided on the Ribosomal204 Database Project (Cole et al., 2009) to identify nearest neighbours.205206 Results and Discussion 207 Sediment Characteristics208 The sediment was dominated by quartz, feldspars (albite and microcline), and sheet silicates209 (muscovite and chlorite). The sediment had a high Si content (36.3 wt %) and contained Al210 (6.77 %), Fe (3.71 %), K (2.67 %), Na (0.92 %), Mg (0.75 %), Ti (0.38 %), Ca (0.27 %), and211 Mn (0.09 %). The total organic carbon content of the sediment was 0.69 % and total carbon212 was 1.70 %. The concentration of 0.5 N HCl extractable Fe in the sediment was213 17.1 ± 1.6 mmol kg-1 prior to incubation and the sediment pH was ~ 5.214215 Neptunium behaviour during progressive bioreduction 216 Manganese enriched (2 mM δMnO2) sediment microcosms amended with electron donor, and217 a Mn enriched sterile control microcosm, were spiked with 3.2 µM Np(V) and incubated over218 a 38 day period. Modelling of the initial groundwater chemistry in PHREEQC-2 (Specific219 Ionic Theory (SIT) database) predicted that the speciation of the Np in solution would be+ 220 predominantly NpO2 (see supporting information S1). In the sterile-control system, the pH221 remained stable and bulk biogeochemical changes indicative of terminal electron acceptor222 progression were not observed (Figure 1 A-F). A release of Mn (~0.2 mM) into the223 groundwater of the sterile control occurred when the sediments were autoclaved (Figure 1C)224 which is similar to past studies with this material (Thorpe et al., 2012). In the microbially-225 active Mn rich microcosms, terminal electron accepting processes progressed in the order- 2- 226 NO3 > Mn > Fe(III) > SO4 reduction as expected (Figure 1B-E). In all microcosms, the pH- 227 remained circumneutral (Figure 1F), NO3 decreased to < 0.2 mM within 11 days, and 11- 228 porewater Mn increased to between 0.05 – 0.1 mM after 9 days suggesting concomitant NO3229 and Mn reduction in these systems (Figure 1B and C). Microbially-mediated Fe(III)230 reduction was then evident after 17 days as indicated by 0.5 N HCl extractable Fe(II)231 ingrowth to sediments. Importantly, in this system Mn reduction (indicated by Mn(II) in232 porewaters) occurred independently of any measurable Fe(III) reduction (indicated by a lack233 of Fe(II) ingrowth to sediments) across three time-points (days 7 to 11; Figure 1). The234 addition of 2 mM δMnO2 to sediments then allowed an extended Mn-reducing ‘period’,235 distinguished from the Fe(III)-reduction, so that Np-behaviour could be tracked throughout236 the stages of early metal reduction. In the sterile control the added Np(V) was partially sorbed237 to the sediment, with ~ 22 % removed from solution after 7 days (Figure 1A). Thereafter, the238 concentration of Np in solution remained stable. Neptunyl sorption has been observed to a239 similar extent as in earlier studies using comparable sediment systems (Law et al., 2010) and240 has been attributed to sorption to negatively charged mineral surfaces (e.g. Fe(III)- or Mn-241 bearing minerals: Combes et al., 1992; Nakata et al., 2002; Arai et al., 2007; Müller et al.,242 2015; Wilk et al., 2005). In the microbially active microcosms prior to the onset of Mn243 ingrowth to porewaters (0 - 7 days), 43.0 ± 1.9 % of the added Np was removed from the244 groundwater (Figure 1A). By day 11 where Mn-reducing conditions had developed, 86.0 ±245 4.9 % of the added Np had been removed from solution. By the end of the experiment,2- 246 following Fe(III) and subsequent SO4 reduction at 38 days, 96.1 ± 0.5 % of the added Np247 was removed to the sediment. Enhanced removal of Np in active systems, compared to the248 sterile control, as observed in the first 7 days and prior to the onset of Mn reduction, could be249 attributed to either reduced surface reactivity in autoclaved sediments and / or enhanced Np250 sorption in the system with microbial cells present (Gorman-Lewis et al., 2005; Ams et al.,251 2013). Results then show a clear relationship between Np(V) removal from solution and Mn252 reduction and confirms that Np(V) is significantly removed from groundwater under Mn- 12253 reducing conditions. It remains unclear in these low concentration Np microcosm studies254 whether Np(V) removal is linked to microbial metabolism or the result of abiotic reaction255 with reduced Mn minerals produced by microbes. The formation of Np-carbonatohydroxo256 complexes has been shown to increase the solubility of Np(IV) (Kitamura and Kohara, 2002;257 Kim et al., 2010). However, in these systems under end-point sulfate reducing conditions (pE258 -4), and taking into account the increase in inorganic carbon expected from acetate utilisation259 (2 mM), solution modelling in PHREEQC-2 (SIT database) predicted that Np would be260 speciated as Np(OH)4 (see supporting information S2). 261262 Neptunium LIII-edge XAS Experiments263 To assess the speciation of Np in sediment microcosms under different biogeochemical264 conditions, select samples were run at the elevated concentrations required for XAS analysis265 (oxic, nitrate-, Mn-, Fe(III)-, sulfate-, progressive Mn- and sterilised Mn- reducing- 266 microcosms). The XANES of Np in the sterile, oxic control sediment and in the NO3267 reducing sediment both showed a Np(V)-like spectra displaying the characteristic multiple268 scattering resonance structure at the high energy flank of the white line resulting from269 scattering along the axial oxygen atoms of the linear neptunyl moiety (Figure 2; Moyes et al.,270 2002; Denecke et al., 2005). Removal of Np from solution in both the oxic and nitrate271 reducing systems at circumneutral pH is occurring and is likely due to Np(V) sorption to Fe272 or Mn mineral surfaces (e.g. Combes et al., 1992; Nakata et al., 2002; Wilk et al., 2005; Arai273 et al., 2007; Law et al., 2010; Müller et al., 2015). By contrast, the XANES spectra for the2- 274 progressive Mn-reducing microcosm, and the poised Mn-, Fe(III)-, and SO4 - reducing275 systems showed Np(IV)-like features with a loss of the multiple scattering resonance276 structure due to the loss of the two neptunyl dioxygenyl oxygen backscatterers (Figure 2).277 Here, the enhanced removal of Np from solution compared to the oxic or nitrate-reducing 13278 systems is attributed to reductive precipitation of Np(V) to Np(IV) (Law et al., 2010). In the279 absence of defined (matrix-matched) standards for these complex systems, linear280 combination fitting of the microbially-active Mn-reducing systems (both progressively281 reduced, and poised), using the oxic sediment and the sulfate-reducing sediment as end-282 members, indicated that Np(IV) was indeed the dominant oxidation state in both systems283 (>90 % Np(IV)) (Ravel et al., 2005). In contrast, the XANES spectra for the sterile Mn-284 reduced microcosm was Np(V) like (> 90 % Np(V)). Interestingly, the presence of significant285 Np(IV) (~90 %) in the microbially-active Mn-reducing systems and dominant Np(V) (~ 90286 %) in the sterile Mn-reducing sediments suggests that microbial reduction of Np(V) to287 Np(IV) is significant in reductive immobilisation of Np(V). Any artefacts associated with288 mineral reactivity and reducing capacity of the sterile Mn-reducing sediment resulting from289 autoclaving cannot however be ruled out here, but the significant change in Np(V) reduction290 between the microbially active and sterile sediments suggests enzymatic processes are likely291 to play a role in controlling Np(V) reductive immobilisation. These observations on sterile292 Mn-reducing sediments differ from those observed in a sterilised Fe(III)-reducing sediment293 reacted with Np(V), which facilitated Np(V) reduction (Law et al., 2010).2- 294 EXAFS data were also collected from the sterile oxic and microbially-active Mn-, and SO4 -295 reducing samples. The k3-weighted EXAFS spectra and their Fourier transform spectra are296 shown together with the corresponding best model fits (Figure 3; Table 1). The best fit to the297 sterilised oxic control sample was a Np(V)-like coordination environment with two axial298 oxygen backscatterers at 1.86 Å and four equatorial oxygen backscatterers at 2.45 Å (Table 1,299 Figure 3). The distances for both axial and equatorial oxygen backscatterers are within the300 range reported for Np(V) (1.82 – 1.88 Å; Combes et al., 1992; Moyes et al., 2002; Denecke et301 al., 2005; Arai et al., 2007; Herberling et al., 2007; Law et al., 2010). The statistical302 relevance of additional shells, containing Fe, Mn, or Np, was assessed using an F-test (Ravel 14303 et al., 2005) and it was found that no significant improvement to the model fit could be304 achieved. The sulfate reducing sample was modelled using a Np(IV) like coordination305 environment with eight oxygen backscatterers at a distance of 2.34 Å (Table 1, Figure 3;306 Llorens et al., 2005; Law et al., 2010). In agreement with the linear combination fitting, the307 best fit for the microbially active Mn-reducing sample was a Np(IV) like coordination308 environment, like the sulfate reducing system, with eight oxygen backscatterers at a distance309 of 2.31 Å. As with the oxic sediment sample, the addition of a second shell of Fe, Np or Mn310 did not significantly improve the fit when using the F test as a measure of validity. Finally, it311 is noteworthy that in the samples where significant Np(IV) was present and where EXAFS312 was possible (the Mn-reducing sediment and sulfate-reducing microbially active sediments313 (Figure 3)), there was no evidence for a Np – O – Np interaction of the type that would be314 expected for nano-particulate NpO2, which has recently been observed in environmentally315 relevant systems (Husar et al., 2015). These observations are in agreement with past work in316 sediment systems (Law et al., 2010) which do not show significant evidence for a Np – O –317 Np interaction. 318319 Microbial community analysis320 Analysis of the microbial community in the Mn-reducing systems with low (3.2 µM) and321 high (0.2 mM) Np content was performed to provide insight into the toxicity of Np(V) in322 these systems. The biogeochemical trajectory was similar in sediments with both low and323 high concentrations of Np, suggesting that the toxicity of Np in these systems was not324 significant. These observations were supported by the RISA results, which were similar325 across both low and high concentration Np amended metal-reducing samples, and clone326 libraries for the same samples, again confirming broadly similar communities (Figure 4).327 Both high and low concentration Np experiments did however show a decrease in 15328 biodiversity compared to the oxic sediment sample (clone libraries from the oxic sediment329 showed 34 operational taxonomic units (OTUs) from 71 clones whereas the biostimulated330 samples showed 7 OTUs from 92 clones in the low Np sample and 12 OTUs from 74 clones331 in the high Np sample). The clone libraries of both low and high Np concentration Mn-332 reducing samples were dominated (> 50 %) by members of the class Bacillus, including333 known denitrifying species consistent with the microcosms being primed with 2 mM nitrate334 prior to Mn reduction. 335336 Conclusions337 Overall these data show that microbially-mediated Mn reduction can lead to reductive338 immobilisation of Np(V) to Np(IV). The addition of bioavailable δMnO2 provides a useful339 approach for prolonging microbial Mn reduction and allowing discrimination between the340 impacts of microbially-mediated Mn and Fe(III) reduction on radionuclide biogeochemistry.341 Removal of Np during Mn reduction was further maintained during Fe(III) and sulfate342 reduction and near complete removal of Np from solution had occurred by the onset of343 sulfate reduction. XANES data confirmed reduction to Np(IV) when Np(V) was exposed to344 microbially active Mn-, Fe(III)-, and sulfate-reducing sediments. Thermodynamically, Mn345 reduction is a more favourable process than Fe(III) reduction and so is likely to occur prior to346 Fe(III) reduction in subsurface environments where electron donor is limited. Although not347 conclusive these results imply that Np(V) reduction in these systems occurs in the presence of348 a viable microbial population rather than abiotically, through reaction with reduced Mn349 bearing minerals. Reduction of Np(V) by metal-reducing bacteria may provide an additional350 mechanism for Np(V) removal from groundwater ahead of the development of robust Fe(III)-351 reducing conditions. Results show the importance of subsurface microbial manganese 16352 cycling on the speciation of neptunium. 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Error bars represent 1σ experimental uncertainty from522 triplicate microcosm experiments (where not visible error bars are within symbol size).523524 Figure 2. Np LIII–edge XANES spectra for Np amended sediments under different biological525 conditions. Spectra from oxic sediment, nitrate-reducing sediment and sterilised Mn-reducing526 sediment samples show the Np(V) like multiple scattering resonance structure resulting from527 high energy scattering along the axial oxygen atoms of the linear neptunyl moiety. Spectra528 from Mn-, Fe(III)-, sulfate- and progressive Mn-reducing sediment samples do not contain529 this feature and are therefore more typical of Np(IV) XANES. 530531 Figure 3. EXAFS spectra and Fourier transforms (uncorrected for phase shift of532 backscattering atoms) for Np on sediments under different geochemical conditions. From top533 to bottom: oxic, Mn- and sulfate-reducing conditions. Black lines are k3-weighted data and534 grey lines are the best model fits to the data.535536 Figure 4. Microbial community analysis of: (A) oxic sediment at 0 days; (B) Mn-reducing537 sediment amended with 3.2 µM 237Np; (C) Mn-reducing sediment amended with 0.2 mM538 237Np. 24539 Table 1 540 EXAFS modelling of Np LIII edge spectra for Np associated with sediments under different 541 biogeochemical conditions. 2 2 2 Path Type CN R (Ǻ) σ (Ǻ ) Xv ROxic 1 O 2 1.86 0.006 59.8 0.0079 2 O 4 2.45 0.021 Mn 1 O 8 2.31 0.014 217. 0.0130 Red 3Sul 1 O 8 2.34 0.012 98.9 0.0227 Red 542 CN is the coordination number, R is the interatomic distance, σ2 is the Debye-Waller factor 2 2 543 (Ǻ ), Xv reduced chi square value and R is the least squares residual and is a measure of the 544 overall goodness of fit. 545546547548549 25550 Figure 1551552 26553554555556557558559560561562563564565 Figure 2 27566567568569570571572573574575576577578579 28580581 Figure 3582583584585586587588589590591592593 Figure 4 29594595596

Neptunium and Manganese Biocycling in Nuclear Legacy Sediment Systems

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